This invention relates to a speed control apparatus for an elevator which corrects the secondary resistance of an induction motor in consideration of the temperature state thereof and can perform a speed control of high precision.
Heretofore, a conventional speed control apparatus for an elevator as shown in FIG. 5 has been proposed (Japanese patent application Laid-open No. 56-123795). In this speed control apparatus for an elevator, an inventer device for driving an A-C motor is employed for controlling the operation of the A-C motor. Referring to FIG. 5, numeral 1 designates an A-C power source of three-phase alternating current, numeral 2 a thyristor converter which converts the alternating current from the A-C power source 1 into direct current, and numeral 3 a smoothing capacitor which smooths the output voltage of the thyristor converter 2. The direct current produced by the thyristor converter 2 as well as the smoothing capacitor 3 is applied to a transistor inverter 4, in which the direct current is inverted into an alternating current. This alternating current is supplied as a primary current to an induction motor 6 for moving a cage, and is detected by a current detector 5.
Shown at numeral 7 is a suspension pulley, which is connected to the rotary shaft of the induction motor 6 through a speed detector 8 so as to be freely rotated by the operation of the induction motor 6. Wound round the suspension pulley 7 is a traction rope 11 to which the aforementioned cage 9 and a counterweight 10 are fixed. The rotation of the suspension pulley 7 runs the cage 9.
Numeral 12 indicates a pattern generating device which outputs a speed pattern command. Numeral 13 indicates a microcomputer, into which the output of the pattern generating device 12 and the output of the speed detector 8 are fed through an interface circuit. A PWM (pulse width modulation) circuit 14 compares an output from the microcomputer 13 and an output from the current detector 5, and subjects the difference to the PWM. Thus, it applies a control signal to the base of the transistor of each phase in the transistor inverter 4 so as to control the primary current.
With the speed control apparatus for an elevator as described above, when the cage 9 of the elevator is to be run, the microcomputer 13 first executes a calculation for the PI (proportional-plus-integral) control of the induction motor 6 on the basis of the output of the pattern generating device 12 and that of the speed detector 8 and evaluates a torque command. Further, in order to perform the so-called vector control in which the magnetic flux of the induction motor 6 is held constant, the microcomputer 13 calculates the command value of the primary current in accordance with equations to be mentioned below and converts it into an analog signal which is applied to the PWM circuit 14.
More specifically, letting T.sub.e denote the torque command obtained by the PI control calculation of the microcomputer 13, a slip frequency command .omega..sub.s is evaluated as: ##EQU1## A torque current component I.sub.T is evaluated as: ##EQU2## Here, m: number of phases, P: number of poles, Lhd O: excitation inductance, l.sub.2 : secondary leakage inductance, r.sub.2 : secondary resistance, and I.sub.E : excitation current.
Accordingly, the command values of the primary currents which are output by the microcomputer 13 are given as follows: ##EQU3##
As described above, the prior-art speed control apparatus for the elevator has calculated the command value of the primary current with the resistance r.sub.2 of the secondary coil of the induction motor 6 deemed constant. In practice, however, the resistance r.sub.2 of the secondary coil of the induction motor 6 changes greatly depending upon temperatures. This has incurred the problem that, when the temperature of the secondary coil is low, the induction motor comes to have an insufficient torque, whereas when it is high, the motor is overexcited, so a speed control suited to an actual operation cannot be performed.